The present invention relates to a method for producing fine-grained alloys, particularly fine-grained 6xxx aluminum alloys which exhibit superplasticity, and to the alloys produced by the method.
The advantages of superplastic properties in metals are well known, and particularly well employed in the automotive and aerospace industry. Because of their fine-grained microstructures, superplastic metals and alloys may exhibit from several hundred percent to several thousand percent elongation without necking when pulled in tension at temperatures 20 exceeding 0.5 Tm, where Tm is the absolute melting temperature of the material. In contrast, non-superplastic metals and alloys typically elongate less than 100% before necking under similar conditions. Accordingly, superplastic metals may be formed into a multitude of complex shapes not achievable with other metals.
Currently, commercial interest in the aerospace and automotive industries is focused on superplastic forming (xe2x80x9cSPFxe2x80x9d). SPF is a manufacturing process which exploits the phenomenon of superplasticity by using low gas pressures (less than about 1000 psi (7 MPa)), and concomitantly low energies, to form parts having complex shapes. This process reduces part counts and the need for fasteners and connectors, reducing product weight and manufacturing costs. In addition, SPF may be performed using a single surface tool in a single forming operation, thus reducing tooling costs. The advent of SPF therefore increases the potential commercial applications in which superplastic materials may be employed.
Superplastic behavior in metallic alloys may be described by the equation
"sgr"=k{acute over (xcex5)}m
where "sgr"=flow stress, k=material constant, {acute over (xcex5)}=strain rate, and m=strain rate sensitivity. In superplastic metals, m usually ranges from about 0.4 to 0.8. xe2x80x9cQuasi-superplasticxe2x80x9d metals and alloys have m values of around 0.33. Materials having m values less than 0.3 are considered to be non-superplastic.
Most metals and alloys capable of achieving superplasticity must be specially processed for superplasticity. The microstructures of such metals and alloys may be refined through thermomechanical processing to impart such properties to the material. For a material to be superplastic, it is typically refined to possess an equiaxed, fine-grained structure, typically with grains about 20 xcexcm or less in diameter and preferably about 10 xcexcm or less. In addition, for such a material to be commercially useful, it must be statically stable such that its grains do not experience significant growth at superplastic forming temperatures. Where the thermomechanical process for refinement includes static recrystallization, which is a common component of such processes, a weak or random texture and the presence of predominantly high-angle grain boundaries is also required. The development of thermomechanical processes effective for creating alloys having such properties has proven to be extremely challenging.
An extensive amount of research has been conducted in an effort to discover thermomechanical processes useful for producing superplastic alloys, including aluminum alloys. This work has resulted in the development of several superplastic alloys, but undoubtedly, many commercially important superplastic alloys have yet to be discovered. In particular, although several superplastic 2xxx, 5xxx, 7xxx and 8xxx aluminum alloys have been produced, there has been a significant deficiency in successful research concerning the grain refinement and superplasticity of 6xxx aluminum alloys. New superplastic 6xxx aluminum alloys would be particularly desirable, because 6xxx alloys are highly weldable, corrosion resistant, extrudable and low in cost compared with other aluminum alloys. Thus, there is a need for the development of methods for imparting superplastic properties to alloys, particularly 6xxx aluminum alloys.
Of the 6xxx aluminum alloys, 6061, 6063, 6066, and especially 6013 and 6111, possess substantial promise for extensive use in the aerospace and automotive industries. Indeed, non-superplastic aluminum alloy 6013, a medium strength, age-hardenable alloy developed by ALCOA in the early 1980s, has been selected for use on Boeing Co.""s state-of-the-art 777 aircraft, as well as for many other automotive and aerospace applications. This is not surprising, given the favorable properties of this alloy and the fact that it can be processed to develop properties superior to other 6xxx alloys. For example, it has corrosion resistance superior to that of 2xxx and 7xxx aluminum alloys, which are heavily used for aerospace applications. The yield strength of 6013-T6 is 12% higher than that of 2024-T3, it is nearly immune to corrosion that results in exfoliation and stress-corrosion cracking, and it is 25% stronger than 6061-T6. In addition, the alloy 6013-T4 has better stretch-forming characteristics than other aerospace aluminum alloys. Accordingly, there is a need for the development of methods for imparting superplastic properties to 6061, 6063, 6066 alloys, and particularly to 6013 and 6111 aluminum alloys.
To date, efforts expended to impart superplasticity to 6xxx aluminum alloys have not been very successful. U.S. Pat. No. 4,092,181 to Paton, et al., which describes what is known in the art as the xe2x80x9cRockwell process,xe2x80x9d discloses a method for imparting a fine grain structure to aluminum alloys having precipitating constituents. The thermomechanical process of the Paton, et al. method consists of solution heat treating such an alloy, overaging the alloy, then subjecting the alloy to a particle-stimulated nucleation (xe2x80x9cPSNxe2x80x9d) process during which the alloy is mechanically worked and recrystallization is induced. Although the Paton, et al. patent provides several examples of the method described therein, it does not describe the microstructures produced by the method, nor does it suggest that superplastic results were achieved. Indeed, experimental evidence available in the literature indicates that the method disclosed by Paton, et al. is not very useful for imparting superplasticity to 6xxx alloys. This is confirmed by the work performed in connection with the present invention, as described below.
Similarly, Washfold, et al. attempted to grain refine a 6063 aluminum alloy through PSN in order to induce superplasticity. See Washfold, et al., xe2x80x9cThermomechanical Processing of an Alxe2x80x94Mgxe2x80x94Si Alloy,xe2x80x9d Metals Forum (1985) at 56-59. The thermomechanical process used is very different than that employed in the present invention, and consists of a solution heat-treatment followed by slow cooling to an overaging temperature, overaging, slow cooling to room temperature, cold or warm rolling, and static recrystallization with a slow heat-up to the recrystallization temperature. Washfold, et al. produced a microstructure exhibiting a minimum grain diameter of 10.5 xcexcm (in the rolling plane), as measured using optical microscopy (xe2x80x9cOMxe2x80x9d) techniques. They obtained a maximum elongation of 148% at 450xc2x0 C., due to significant grain growth occurring at 500xc2x0 C. and above, within the superplastic forming temperature range. The Washfold, et al. process did not achieve superplasticity.
Kovacs-Csetenyi, et al. attempted to use compositional variation and thermomechanical processing to refine the grain structure and improve the superplastic performance of aluminum 6066 and three variants of aluminum 6061. See Kovacs-Csetenyi, et al., xe2x80x9cSuperplasticity of AlMgSi Alloys,xe2x80x9d Journal of Materials Science 27 (1992) at 6141-45. The thermomechanical process used consists of solution heat-treatment followed by overaging, rolling, and static recrystallization, and bears no resemblance to that of the present invention. Kovacs-Csetenyi, et al. report strain rate sensitivity values in the range of 0.4 for each of the four alloys processed, as studied using temperatures between 500xc2x0 C. and 570xc2x0 C. and strain rates of 10xe2x88x923 to 10xe2x88x926 sxe2x88x921, indicating that some degree of superplastic behavior would be expected from the alloys. However, superplasticity was characterized using impression creep tests, and no uniaxial tensile tests were reported. Thus, it is unclear what amounts of superplastic elongation, if any, were obtained by the processing technique described in this reference.
Chung, et al. also experimented with grain refinement techniques to produce a superplastic 6013 alloy. See Chung, et al., xe2x80x9cGrain Refining and Superplastic Forming of Aluminum Alloy 6013,xe2x80x9d The 4th International Conference on Aluminum Alloys (1994), 434-42. Chung, et al. employed a thermomechanical process consisting of solution heat-treatment, 10% cold rolling, overaging at 380xc2x0 C., 90% warm rolling at 190xc2x0 C., and recrystallization. In contrast to the process of the present invention, Chung, et al. employed mild cold rolling, for the purpose of forming a dislocation network to assist in the precipitation of what was thought to be Mg2Si precipitates. The process resulted in grains of 12 to 13xcexcm (measured using optical microscopy techniques), a strain rate sensitivity of 0.38, and a maximum elongation of 230% at 520xc2x0 C. for a strain rate of 3xc3x9710xe2x88x924 sxe2x88x921, and at a flow stress of 972 psi (6.7 MPa). Thus, the product of the Chung, et al. process was only marginally superplastic. Chung, et al. concluded that the size and number of iron-bearing constituents in the alloy needed to be reduced in order to achieve more favorable results. Chung, et al. clearly were not aware that, as disclosed by the present invention, a significantly higher energy deformation structure such as a deformation band needed to be imparted to the material and exploited to form sites for the heterogeneous nucleation of precipitates, enabling the achievement of a superplastic microstructure.
A similar process to that employed by Chung, et al., but directed to an altogether different purpose, is described in U.S. Pat. No. 3,706,606 to DiRusso, et al. The DiRusso patent addresses the need to develop processes for increasing the mechanical strength of semifinished aluminum alloys. Like Chung, et al., the DiRusso patent describes using a mild cold or warm rolling between solution heat-treatment and aging steps to provide a dislocation network to assist in precipitation. None of the alloys treated using the process of the DiRusso patent exhibited superplastic properties, as shown by the tensile elongation tests performed by DiRusso, et al. on such alloys, nor were they intended to do so.
Accordingly, it is an object of the present invention to provide alloys exhibiting superplasticity, particularly 6xxx alloys and especially aluminum 6013 and 6111 alloys.
It is another object of the present invention to provide a method for imparting superplastic properties to alloys that is applicable to a wide range of alloys, particularly all 6xxx alloys and especially aluminum 6013 and 6111 alloys.
It is yet another object of the present invention to provide a method for imparting superplastic properties to alloys that is economical and commercially useful.
It is still another object of the present invention to provide a method for producing superplastic alloys having an equiaxed, uniform, thermally stable, fine grain structure of less than about 20 xcexcm, and preferably about 10 xcexcm or less.
It is another object of the present invention to provide a method for producing superplastic alloys having a microstructure with a weak or random texture and a predominance of high-angle grain boundaries.
In accordance with the principles of the present invention, alloys exhibiting superplasticity and a method for producing the same are provided. The method involves inducing in an alloy the formation of precipitates having a sufficient size and homogeneous distribution such that, after a subsequent PSN process, a sufficiently refined grain structure to produce superplasticity results. The process of the present invention differs from previous processes in the particular thermomechanical processing steps required, as well as in the sequence and character of those steps. Because of these differences, the process of the present invention is capable of imparting to age-hardenable alloys, and particularly to age-hardenable aluminum alloys, exceptional superplastic characteristics heretofore not obtainable. An exemplary alloy of the type capable of being produced by the present invention is a superplastic 6xxx alloy which is economically produced and commercially useful for aerospace, automotive and other applications.
The method for producing a superplastic alloy, as provided by the present invention, comprises providing an age-hardenable alloy for processing which has a matrix phase and at least two alloying elements, at least one of the alloying elements being, or being capable of forming, a dispersoid phase substantially insoluble in the matrix phase after basic ingot processing. The alloy is solution heat-treated, and cooled to form a supersaturated solid solution. The alloy is then plastically deformed sufficiently to form a high-energy defect structure, thereby forming nucleation sites useful for the subsequent heterogeneous nucleation of precipitates. The alloy is then aged, forming precipitates at the nucleation sites, and subjected to deforming and recrystallizing through a PSN process.
This process has been shown to effect excellent results in a variant of an aluminum 6013/6111 alloy, but is suitable for processing any age-hardenable alloy. Aluminum alloys, particularly 6xxx aluminum alloys, and more particularly 6013, 6111,6061, 6063 and 6066, are particularly good candidates for processing under the present method.
The cooling step following solution heat-treatment may be performed using any mode of rapid cooling. For example, it may be performed by quenching in media such as water, oil or air. The step of plastically deforming the alloy must be sufficiently severe to form a high-energy defect structure, such as the high-energy defect structures commonly referred to as xe2x80x9cdeformation bands,xe2x80x9d in contrast to lower-energy defect structures such as a dislocation network. Such severe plastic deformation may be imparted by any means, such as a rolling, stretching, extrusion, drawing, forging or torsion process at economical temperatures and conditions, and is preferably imparted by cold rolling at room temperature.
The aging process of the present invention may comprise a single heating step in which the alloy is heated at a single temperature for a set period of time, or multiple heating steps in which the alloy is heated at different temperatures over set time periods. Preferably, the aging process comprises a first heating step at a first temperature and a second heating step at a second higher temperature. The first heating step may be used to form the precipitates, which then may be coarsened during the second heating step. Where two or more heating steps are used, the alloy preferably is cooled after each heating step.
The PSN process preferably includes plastically deforming the alloy to provide sufficient strain energy in the alloy to ensure recrystallization, and statically recrystallizing the alloy. The plastic deformation step of the PSN process may include any mode of plastic deformation, but preferably comprises cold rolling the alloy at room temperature. The static recrystallization step of the PSN process preferably includes rapidly heating the alloy to a temperature at which recrystallization occurs and at which recovery is minimized. In one embodiment, such rapid heating is provided by selecting a recystallization temperature in the range of the solution heat-treatment temperature for the alloy. In another embodiment, rapid heating is provided by heating the alloy to the superplastic forming temperature of the alloy.
One of the alloys which may be processed to exhibit exceptional superplastic properties using the method of the present invention is a 6013/6111 aluminum alloy having the approximate composition 97.3 wt % Alxe2x80x940.8 wt % Mgxe2x80x940.7 wt % Sixe2x80x940.8 wt % Cuxe2x80x940.3 wt % Mnxe2x80x940.1 wt % Fe. In one embodiment of the present invention, the solution heat-treating step is performed by heating this alloy at a temperature of about 540xc2x0 C. for about one hour, excluding heat-up time. The solution heat-treated alloy is then rapidly cooled, preferably by cold water quenching. The alloy is then plastically deformed to a sufficient degree to form the required deformation bands or other high-energy defect structures in the material. This may be done, for example, by cold rolling at room temperature by about 30% or more. Most preferably, the plastic deformation is performed such that, after subsequent aging, the alloy will exhibit a uniform distribution of globular or near-spheroid shaped precipitates. Aging may be performed using any combination of aging steps, but preferably is performed using a two-step aging process. In one embodiment of the invention, a first heating step is performed at about 300xc2x0 C. for about 24 hours and a second heating step is performed at about 380xc2x0 C. for about 24 hours, with the alloy being cooled after the each of the heating steps. Precipitates preferably are formed during the first heating step and coarsened during the second heating step.
According to another exemplary embodiment, the 6013/6111 superplastic aluminum alloy of the present invention may be aged using a first heating step at about 300xc2x0 C. for about 24 hours, and a second heating step at about 450xc2x0 C. for about 2 hours. Under yet another exemplary embodiment, the alloy may be aged using a single heating step, at a temperature of about 450xc2x0 C. for about 2 hours. Although the microstructure of this single-heating step alloy may be somewhat less ideal than those of the alloys produced using the dual heating steps of the other exemplary embodiments, such a low temperature/short heating time process may be preferred for commercial applications where energy consumption and time are important factors.
After aging, the 6013/6111 aluminum alloy of the present invention is plastically deformed to provide sufficient strain energy in the alloy to ensure recrystallization. In one embodiment of the invention, the alloy is cold rolled at room temperature by about 80% or more. In particular, cold rolling at room temperature by about 80%, 87% and 92% has produced exceptional results. Smaller amounts of plastic deformation may also be employed. The alloy is then recrystallized. In connection with the recrystallization step, the alloy should be rapidly heated to the temperature at which recrystallization occurs to minimize recovery within the deformation zones around the precipitates and to activate the largest number of recrystallized nuclei. In one embodiment of the invention, the alloy is rapidly heated to about 540xc2x0 C. and held there for about five minutes.
Processing the 6013/6111 aluminum alloy as discussed yields a superplastic alloy with a microstructure having a fine average grain size in the range of about 9.5 xcexcm to about 11.6 xcexcm, the grain sizes having a standard deviation in the range of about 4.7 xcexcm to about 5.6 xcexcm. In addition, the mlcrostructure of the alloy has a low average grain aspect ratio (i.e., ratio of major axis to minor axis) in the range of about 1.6 to about 1.9, the grain aspect ratios having a standard deviation in the range of about 0.6 to about 0.8. The alloy also has a grain roundness in the range of about 1.6 to about 1.8, a maximum strain rate sensitivity of at least about 0.5, and a maximum elongation capability of at least about 350%, preferably 375% or more. Specifically, in one embodiment, processing the 6013/6011 alloy using a first heating step at about 300xc2x0 C. for about 24 hours and a second heating step at about 380xc2x0 C. for about 24 hours, with the alloy being cooled after the each heating step, and subsequently cold rolling the aged alloy by about 87% and recrystallizing the alloy at about 540xc2x0 C. for about five minutes, yields an average grain size of about 9.5 xcexcm (about 4.7 xcexcm standard deviation), and an average grain aspect ratio of about 1.6 (about 0.6 standard deviation). The resulting alloy has a maximum strain rate sensitivity of about 0.5 at 540xc2x0 C. for a strain rate range of 2xc3x9710xe2x88x924 sxe2x88x921 to 5xc3x9710xe2x88x924 sxe2x88x921, and a maximum elongation of 375% with a corresponding maximum stress of approximately 680 psi (4.7 MPa).